Modelling Realistic Paper Poster With Curl?
I'll expand my comment above, to post an example. 

As I said, if you literally mean "like the picture below", well that paper is perfectly square and not bent in any way, imho.

This is a typical fake effect for 2d apps or web sites. The shadow is simulated as if the surface below was detached from the paper in the upper and lower part, but the paper is not curved as the shadow "visually suggests". 

It is more like if the below surface was a bit curved, like a cylinder, if you wish your "paper sheet" to be perfectly square and flat, as in the image, you need to curve the surface, not the paper, and use a light source not perfectly centered, to avoid shadow symmetry.

Here is a possible setup, in the image below ( the render was with slightly grey paper, I just later changed the paper color to blue, to make it stand out in the setup on the right side).



But if you really need the paper to be curved, and the surface flat, see other answers.